1. Field of the Invention
The present invention relates to an optical amplifier designed to compensate for the optical power loss and/or wavelength dependency output power difference occurring during transmission of a WDM (Wavelength Division Multiplexing) signal.
2. Description of Related Art
When a WDM signal is transmitted over long distances, optical amplifiers are introduced into transmission lines in order to compensate for the optical power loss in the transmission lines. Systems obtained by the multistage connection of optical amplifier units are often used as such optical amplifiers.
On the other hand, when a WDM signal is transmitted over a transmission line, nonlinear optical effects will occur in the transmission line if the WDM signal is too powerful.
DCFs (Dispersion compensating fibers) or FBGs (Fiber Bragg gratings) are commonly introduced into transmission lines in order to inhibit the formation of such nonlinear optical effects.
Both DCFs and FBGs have high optical power loss, however. Consequently, neither DCFs nor FBGs are introduced on the input or output side of an optical amplifier. The reason is that the NF (Noise Figure) increases when a DCF or FBG is introduced on the input side of an optical amplifier, and output decreases when these are introduced on the output side thereof. In view of this, DCFs or FBGs are commonly introduced between an optical amplifier unit and another optical amplifier unit (between stages). For example, in the case of a two-stage optical amplifier comprising a pre(preliminary)-stage optical amplifier unit and a post(subsequent)-stage optical amplifier unit, DCFs or FBGs are introduced between the pre-stage optical amplifier unit and the post-stage optical amplifier unit.
In the case of optical amplifiers for wavelength division multiplexing (WDM) signals, it is important that powerful output be achieved across a wide wavelength band, the optical output power difference be kept small across a wide wavelength band, a low NF be obtained, and the like. However, some of the aspects of conventional optical amplifiers still need improvement because of the following, for example.
A DCF has a wide compensation wavelength bandwidth, but it also has a nonlinear optical limit. As used herein, the term "nonlinear optical limit" refers to the light input conditions under which a nonlinear optical effect is created in a transmission line.
FIG. 12 is a diagram depicting a system comprising a conventional two-stage optical amplifier 15 composed of a pre-stage optical amplifier unit 11 and a post-stage optical amplifier unit 13. When the aforementioned DCF 17 is introduced between the pre-stage optical amplifier unit 11 and the post-stage optical amplifier unit 13, the optical input power for the DCF 17, while varying with the system, must be kept at a maximum level of no more than about +3 dBm per channel. This is the reason that the gain of the pre-stage optical amplifier unit 11 must limited, the output of the prestage optical amplifier unit 11 must be lowered by means of an attenuator, or the like.
In addition, the DCF 17 creates substantial signal transmission loss (optical power loss, approximately equal to several decibels), making it impossible for the optical amplifier 15 to generate high total output. Specifically, high output is difficult to achieve with an optical amplifier 15 obtained using DCF.
Furthermore, the total NF (NF total) of a two-stage optical amplifier 15 can be expressed as NF total=NF1+(NF2/G1), where NF1 is the NF of the pre-stage optical amplifier unit 11, NF2 is the NF of the post-stage optical amplifier unit 13, and G1 is the gain of the pre-stage optical amplifier unit 11. When, however, a DCF 17 is introduced between the pre-stage optical amplifier unit 11 and the post-stage optical amplifier unit 13, the gain of the pre-stage optical amplifier unit 11 becomes (G1-Ld), where Ld is the optical power loss of the DCF 17, and the result becomes NF total=NF1+{NF2/(G1-Ld)}.
Ultimately, introducing a DCF into the intermediate stage of a two-stage optical amplifier 15 limits the gain of the pre-stage optical amplifier unit 11 because of the nonlinear optical limit of the DCF, and appears to further reduce the gain of the pre-stage optical amplifier unit 11 because of the optical power loss caused by the DCF 17. Consequently, the NF of the post-stage optical amplifier unit 13 affects the total NF of the optical amplifier 15.
The performance of the post-stage optical amplifier unit 13 is important for compensating the factors (limited optical input power for the DCF, and the optical power loss originating in the DCF itself) that reduce the output of the optical amplifier 15 depicted in FIG. 12. With the conventional practice of amplifying the entire wavelength bandwidth of WDM signals by a single post-stage optical amplifier unit 13, however, inherent limitations are encountered when attempts are made to achieve high output and low optical output power difference across a broad wavelength band.
On the other hand, an FBG has lower signal transmission loss (optical power loss) than a DCF. In addition, an FBG is free from nonlinear optical limits. Consequently, introducing an FBG (not shown) instead of a DCF between the pre-stage optical amplifier unit 11 and the post-stage optical amplifier unit 13 of a two-stage optical amplifier 15 yields an optical amplifier capable of chromatic dispersion compensation and makes it possible to obtain an optical amplifier in which the signal transmission loss originating in the FBG itself is reduced and in which no limitations are imposed on the optical input power for the FBG.
In the case of an FBG, however, the wavelength bandwidth within which the chromatic dispersion can be compensated for is narrow (for example, about 7 nm), limiting the bandwidth that can be transmitted by an optical amplifier when the FBG is introduced between the stages of the optical amplifier.
A need therefore existed for an optical amplifier in which higher output and lower wavelength dependency output power difference could be achieved across a wider wavelength bandwidth than in the past.